1. Field of the Invention
The present invention relates to a solar energy system designed to detect the operating point of a photovoltaic source so as to be capable of automatically tracking the maximum power point of the photovoltaic source and stably and constantly supplying the maximum power from the photovoltaic source to loads.
2. Description of the Related Art
Conventionally, a system as shown in FIG. 13 is proposed as a solar energy system constructed in a way to obtain the maximum power from a photovoltaic source. This solar energy system SO is constructed by connecting a photovoltaic array P to a stand-alone system load L and other utility system A, respectively, through a DC/DC converter H such as DC/AC inverter, etc. Specifically it is designed to supply the maximum power from the photovoltaic array P to the stand-alone system load L and the other utility system A by being provided with a voltage transfer DS which sets a voltage value enabling output of the maximum power by the photovoltaic array P in response to the output signal from a current sensor D1 which detects the output current of the photovoltaic array P, a voltage detector DV which detects the output voltage of the photovoltaic array P and a regulator RE which compares a voltage value V1 detected by the voltage detector DV with a set voltage value V2 of the voltage command device DS and which controls the output voltage or output current of the converter H (For example, see Japanese Unexamined Patent Publication JPA 56-91630 (1981)).
However, the output characteristics of a photovoltaic cell depend on environmental changes such as atmospheric temperature, solar irradiance, etc., dust on the light receiving face of the photovoltaic cell, yearly declination of the photovoltaic cell, etc. The conventional solar energy system SO described above, which is designed to simply preset a voltage value enabling output of the maximum output by the photovoltaic array P and to control the output of the photovoltaic array P based on this set value, incapable of accurately tracking the maximum power point in response to the fluctuations of characteristics of the photovoltaic cell due to environmental changes, etc. as well as of utilizing the photovoltaic source always at a high efficiency.
Moreover, to solve such a problem, one may propose a method which consists of measuring the temperature on the back face of the photovoltaic cell and determining the operating point of the photovoltaic cell which is variable with temperature fluctuations by referring to the characteristics data of the photovoltaic cell obtained in advance by experiments, etc. or also a method which consists of forcibly moving the operating point of the photovoltaic cell and searching for the maximum power point of the photovoltaic cell by comparing differences between the power produced before the movement and that produced after the movement. However, it is difficult to effectively operate the photovoltaic cell if its characteristics are unknown and, even when characteristics data, etc. obtained by experiments are abundantly available, it is impossible to accurately obtain the maximum power point of the photovoltaic cell in the case where the characteristics of the photovoltaic cell have changed because of yearly declination, etc. or where the characteristics have deteriorated because of dust on the light receiving face, etc.
Especially, in the method of forcibly moving the operating point of the photovoltaic cell and comparing the differences between the power produced before the movement and that produced after the movement, one cannot determine the exact cause of fluctuations in the output of an apparatus in the case where any major changes in the natural environments such as shower or movement of clouds, etc. occur at the time of movement of the operating point, even if it is possible to avoid problems of deterioration of characteristics due to yearly declination, etc. or changes of characteristics due to dust on light receiving face, etc. This makes it impossible to determine the accurate maximum power point. To solve this problem, it is necessary to build complicated logical circuits capable of performing majority logic, fuzzy logic, etc., leading to another problem of the need of not only large equipment size but also extreme complexity of the entire system.